In the case of the transmission systems with variable attenuations of the signal, it is generally desired that the method for transmission of the signal has a high data rate together with a high level of availability over time. Since the data rate is of the form:                spectral efficiency×bandwidth,the idea is therefore to obtain a high spectral efficiency and a high level of availability.        
In the case of interactive or point to point services, this problem is solved using adaptive physical layers: the receivers inform the transmitter of the SNR (Signal-to-Noise Ratio), which can adjust, for example, the modulation and coding scheme of the waveform. This solution may not be envisaged in broadcast mode.
In the future satellite broadcast systems, whether this be to mobiles, or to fixed stations in Ka band (20-22 GHz), it is difficult to ensure a high level of availability and a high spectral efficiency owing to the occurrence of severe attenuations within the channel. In the case of mobiles, these attenuations result from masking effects by the mountainous terrain, the buildings, the vegetation; in the case of broadcasting to fixed stations, in the Ka band, they are due to climatic events such as rain.
One known solution is to provide two levels of quality of service:                a minimal service (HP for High Priority) with a very high level of availability even during the periods of severe attenuations, such as for example a standard definition of the image in the case of video service assured for 99.9% of the year, including during rainy periods and        a nominal service (LP for Low Priority) during favorable periods such as for example for a mobile in the line of sight of the satellite, or in a clear sky for the Ka band for 99% of the year.        
For example, for the broadcasting of HD TV in the Ka band, the minimal TV service is in SD (“Standard Definition”), the nominal service being in HD (“High Definition”).
This solution is obtained by using hierarchical modulation. The prior art on hierarchical modulation is typically that of the standards such as DVB-SH (EN 302 583) or DVB-S2 (EN 302 307).
Hierarchical modulation allows two binary streams, labeled high priority (HP) and low priority (LP), to be made to pass within the same transmission channel, the most significant bits (or MSB) of the constellation for the modulation being reserved for the HP stream after coding, and the least significant bits for the LP stream after coding. The rate of coding for each data stream is adjustable, which allows two different operating points for the high priority stream (HP) and for the low priority stream (LP). It is recalled that an operating point is the SNR required for a target Bit Error Rate (or BER).
By means of a transmitter such as that shown in FIG. 1a, two independent virtual transmission channels are thus created: an HP channel and an LP channel. It comprises a coder 1, 2 for each stream, a hierarchical modulator 20 which comprises means 21 for reserving the most significant bits (or MSB) for the HP stream and means 22 for reserving the least significant bits for the LP stream.
The ratio of the HP and LP rates is fixed by the type of constellation for the modulation. Two examples of constellation diagrams are shown in FIGS. 2a-2c. In the example in FIG. 2a, the hierarchical modulation based on an 8PSK modulation (acronym for the expression “8 Phase Shift Keying”, or modulation by phase shifting over 8 possible values of phase), transmits two HP bits and one LP bit for each symbol; the hierarchical modulation based on a 16-QAM modulation (acronym for the expression “16 Quadrature Amplitude Modulation”, or quadrature amplitude modulation over 16 states), shown in FIG. 2b, transmits two HP bits and two LP bits for each symbol. In FIG. 2b can also be seen the bits coded 00 coming from the coder 1 and the bits 10 coming from the coder 2 in the diagram of the constellation 23. The ratio of the HP and LP rates is therefore highly constrained.
On the receiver side, the basic (HP) stream received can practically always be decoded correctly, but the improved (LP) stream received is only decoded correctly if the conditions are favorable. One conventional design of receiver is shown in FIG. 1b, symmetric case of the transmitter in FIG. 1a. It comprises a hierarchical demodulator 20′ with means 21′ for allocating the most significant bits (or MSB) to a first stream, and means 22′ for allocating the least significant bits to a second stream, two decoders, one 1′ for the stream comprising the most significant bits, the other 2′ for the stream comprising the least significant bits, which at the output are one the HP stream, the other the LP stream.
In FIG. 3 are shown examples of curves of Bit Error Rate (BER) as a function of the SNR, one for an HP stream coded with a very robust coding rate (for example ¼), and an LP stream, more capacitive, coded with a higher coding rate (for example ½). Based on a required quality (target BER), the required SNR is determined for each HP stream, in other words the HP and LP operating points denoted OPHP and OPLP. It can be seen that, for certain ranges of real variation of the SNR in a given system, such as that in FIG. 3, the SNR is too high with respect to the operating point of the HP stream and too low for the LP stream which will therefore never be decoded: the performance in this type of system is then sub-optimal.
The operating points of the two HP and LP virtual channels are, by construction, higher than the operating points of an equivalent real channel having the same spectral efficiency determined as a function of the coding rate (=number of useful bits/(number of useful bits+number of redundant bits)) and of the chosen modulation. It is then a question of penalty.
The penalties for the HP and LP virtual channels are dependent on the geometrical characteristics of the constellation. These consist of an angle of separation (2θ) of the low priority blips in an 8PSK modulation (see FIG. 2a) and of an amplitude ratio (α=dh/dl) in the case of the 16-QAM modulation (see FIG. 2b). This parameter allows the HP and LP penalties to be adjusted.
Finally, the ratio of the HP and LP rates, together with the operating points, are fixed and dependent on the coding rate of the coders for the HP and LP streams, and also on the type of the constellation.
The document EP 2 207 320 provides a static improvement for the performance curve of the LP stream (Bit Error rate as a function of the Signal-to-Noise Ratio). A first coder is used for coding the bits of the HP stream as in a conventional hierarchical modulation. In contrast, a second coder is used for coding the bits of the LP streams and a part or all of the HP bits thus allowing a redundancy to be created between the 2 coders in the HP bits. This redundancy is used during the decoding for improving the operating point of the LP stream (this is a fixed improvement of around 0.6 dB for α=1). However, this static improvement of the LP stream imposes a rate constraint for the LP stream.